xseg: Add X_CREATE op and protocol entry

Extend XSEG protocol with the X_CREATE op and the associated request data.
python: update distribute_setup.py
xseg: Add contaddr_size field to xseg request
xseg: Add rename op and request struct
xseg: Add flags to map reply struct

Add flags to map reply struct.

Introduce a new ZERO flag to mark the zero objects introduced by mapper and do
not exist on the backend.
Merge PR #4 (feature-xcreate) from philipgian/libxseg into develop

This patch set adds support for two new xseg operations:
XCREATE and XRENAME.

It also adds the necessary fields to support the assume_v0
functionality for Archipelago.